1. Field
This invention relates generally to a polyolefin microporous breathable film and method of making same. More specifically this invention is directed toward filled polypropylene microporous breathable films having an improved Water Vapor Transmission Rate (WVTR), high tear strength, high dart impact strength, and a soft feel.
2. Definitions
As used therein the term “extrusion” is intended to include extrusion, coextrusion, extrusion coating, or combinations thereof, whether by tubular methods, planar methods, or combinations thereof.
An “oriented” material is defined herein as a material which, when heated to an appropriate temperature above room temperature (e.g., 96° C.), will have a free shrink of about 5% or greater in at least one linear direction.
Unless specifically set forth and defined or otherwise limited, the term “polymer” as used herein generally includes, but is not limited to, homopolymers, copolymers, such as, for example, block, graft, random and alternating copolymers, terpolymers, etc. and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term “polymer” shall include all possible molecular configurations of the material. These structures include, but are not limited to isotactic, syndiotactic and random molecular configurations.
The term “polyethylene” as used herein refers to families of resins obtained by substantially polymerizing the gas ethylene C2H4. Polyethylenes having densities ranging from about 0.900 g/cc to about 0.935 g/cc are typically called low density polyethylenes (LDPE).
The term “linear low density polyethylene” (LLDPE) as used herein for a type of polyethylene to be employed in the film of the invention, refers to the copolymers comprised of a major amount of ethylene with a minor amount of one or more comonomers selected from C3 to about C10 or higher alpha olefins such as butene-1,4-methyl petene-1, hexene-1, octene-1, etc. in which the molecules thereof comprise long chains with few side chains or branched structures achieved by low pressure polymerization. The side branching which is present will be short as compared to non-linear polyethylenes. The molecular chains of a linear polymer may be intertwined, but the forces tending to hold the molecules together are physical rather than chemical and thus may be weakened by energy applied in the form of heat. Linear low density polyethylene has a density preferably in the range from about 0.911 g/cc, more preferably in the range of from about 0.912 g/cc to about 0.928 g/cc for film making purposes. The Melt Flow Index of LLDPE generally ranges from between about 0.1 to about 10.0 grams per ten minutes and preferably between from about 0.5 to about 3.0 grams per ten minutes. LLDPE resins of this type are commercially available and are manufactured in low pressure vapor phase and liquid phase processes using transition metal catalysts. LLDPE is well known for its structural strength and anti-stress cracking properties. Also, LLDPE is known for its favored properties in the heat shrink process, and thus is well suited to make a heat shrinkable film as discussed above. Also, very low density linear low density polyethylenes (VLDPE) may be employed, and such have a density from about 0.010 g/cc to about 0.860 g/cc, or less.
The term “polypropylene” as used herein which polypropylene is a type of polyolefin that may be employed in the film of the present invention, refers to families of resins obtained by substantially polymerizing the gas propylene, C3H6.
By varying the comonomers, catalysts and methods of polymerization, properties such as density, melt index, crystallinity, degree of branching, molecular weight and molecular weight distribution can be regulated over wide ranges. Further modifications are obtained by other processes, such as halogenation, and compounding additives.